Porting and Borrowing More: How Lenders Split the New Mortgage

Porting does not usually mean moving one unchanged loan from one property to another. You are applying for a new mortgage with the same lender and, if you need to borrow more, the new mortgage can be split into separate parts.

This guide focuses on that split: which balance can keep the old product, what happens to the extra borrowing, and why two different end dates can matter later.

If you move to a more expensive property and need a larger mortgage, a lender may allow you to:

1. port the eligible existing balance onto the old product or rate; and
2. borrow the extra amount on a current product.

That can leave one mortgage account with two or more sub-accounts.

Halifax, Nationwide and other major lenders describe this structure in their current porting guidance. It is still a new mortgage application, so affordability, credit and property criteria apply.

The calculation is:

new purchase price - deposit/equity being used = required new mortgage

Then compare that with the existing mortgage balance eligible to be ported.

For example:

• existing mortgage balance: £150,000
• sale equity available for the new deposit: £100,000
• new property price: £350,000
• required new mortgage: £250,000

If the lender agrees to port the £150,000 balance, the additional borrowing is £100,000.

The new mortgage might therefore look like:

The exact structure is lender-specific, but the principle is common.

Portability normally applies to the balance attached to the existing product, not to unlimited future borrowing.

The extra amount is new lending at today's criteria and available product range. Halifax's current guidance explicitly says additional borrowing can create a second part on a different rate.

This matters when your existing fixed rate is unusually low. You may keep that low rate on part of the debt while paying a much higher or lower current rate on the rest.

Read our full mortgage porting guide if you need the mechanics of porting itself.

This is one of the easiest details to miss.

Suppose:

• the ported £150,000 fix ends in 18 months; and
• the extra £100,000 is placed on a new five-year fix.

You now have two product end dates more than three years apart.

When the first part ends, your options can be constrained because switching only that part to another lender is generally not the same as remortgaging the whole mortgage. You may end up taking a product transfer on one part until the dates align, or paying an early repayment charge on the other part if you want to move everything together.

A slightly more expensive two-year product on the extra borrowing can sometimes produce a cleaner future remortgage date than the cheapest five-year product. That is not automatically better — it is simply one of the costs to compare.

A portable rate does not guarantee a portable application.

The FCA's responsible-lending rules require lenders to assess affordability in the circumstances required by MCOB 11. Because you are asking to borrow more, the extra amount is not simply an administrative transfer.

The lender can reassess:

• income;
• committed expenditure;
• credit commitments;
• dependants and household costs;
• the term;
• the new monthly payment;
• the property; and
• loan-to-value.

If you are close to the lender's affordability limit, read what to do after an affordability decline before making multiple applications.

The lender is taking security over a different property.

That means it can decline or restrict the application because of the new property's:

• value;
• construction;
• condition;
• tenure;
• lease terms;
• location;
• intended use; or
• other lending-policy issues.

Portability is a feature of the existing product, not a waiver of property criteria.

Your new overall LTV affects what additional-borrowing products are available.

For example, if the new mortgage is £250,000 on a £350,000 property, the overall LTV is about 71.4%.

The lender may use that overall LTV when deciding which new products are available for the extra £100,000. A larger deposit could put the case into a different product band.

Our guide to loan-to-value and common LTV bands explains the calculation.

Do not compare only the headline rate on the extra borrowing.

Check:

• product fees;
• valuation fees;
• any porting-related conditions;
• early repayment charges;
• exit or account fees;
• whether the fee is added to the mortgage; and
• the cost over the period you realistically expect to keep each part.

Two sub-accounts can also mean two sets of future product decisions.

Porting can preserve a valuable old rate, but it is not automatically cheapest.

Compare at least two scenarios:

Scenario A: Port + top-up

Calculate the cost of:

• the ported balance on its old rate;
• the extra borrowing on its new rate;
• all fees; and
• any future ERC problem created by mismatched end dates.

Scenario B: New mortgage for the whole amount

Calculate:

• any ERC for leaving the existing mortgage;
• the new rate on the full balance;
• fees; and
• whether the new product gives you a cleaner structure.

The lower rate on one part does not prove the whole transaction is cheaper.

That is a different problem.

If the new mortgage is smaller than the balance on your existing product, the lender may only let you port the amount you still need. The unported balance can be treated as an early repayment and an ERC may apply.

Halifax's current guidance specifically warns that borrowing less can create an ERC on the difference.

First identify what was declined.

It can be:

• affordability;
• creditworthiness;
• the new property;
• the requested extra borrowing;
• the porting feature itself; or
• a lender-specific timing or criteria issue.

Those problems have different solutions. See what to do if a porting application is declined.

Browse all our mortgage guides.

• Halifax — How porting a mortgage works
• Nationwide — Moving home and porting
• FCA Handbook — MCOB 11 responsible lending
• MoneyHelper — Moving home and mortgages

Related: Mortgage Porting Explained | Loan-to-Value Explained | Mortgage Declined on Affordability.